1. Field of the Invention
The present invention relates to an apparatus and a method for the shift control of an automatic transmission and, more particularly, to a shift control apparatus and a shift control method for the shift control of an automatic transmission adapted to control coupling force of a friction coupling element at the time of shifting.
2. Description of Related Art
An automatic transmission generally comprises a torque converter and a transmission mechanism using a planetary gear mechanism which in turn is provided with friction coupling elements for shifting such as a variety of clutches and brakes for shifting power transmission passages in such a manner that they are operable by oil pressure control circuits. The shifting of speed ranges is performed by coupling or uncoupling, or releasing the coupling of, the friction coupling element by means of control of a solenoid valve mounted in the oil pressure control circuits. The switching of the friction coupling elements from a coupled state to an uncoupled or released state or vice verse may cause torque shock unless changes of coupling force are adjusted in an appropriate manner.
In order to solve such a problem, attempts have been made so far to regulate coupling force of friction coupling elements at the time of shifting speed ranges. For example, Japanese Patent Publication (kokai) No. 10,851/1981 (a U.S. counterpart being U.S. Pat. No. 4,283,970) discloses an apparatus in which friction coupling elements are supplied with oil pressures in such a manner as changing coupling force at the time of shifting and concurrently an actual shift time, namely, a period of time required actually for shifting speed ranges, is adjusted so as to correspond to a reference shift value of a shift time set in advance by subjecting the oil pressures to correction control in accordance with a difference between the reference shift value and the actual shift time.
It has been found difficult, however, to adjust changes of torque and numbers of revolutions during the step of shifting speed ranges merely by subjecting the oil pressures fed to the friction coupling elements in accordance with a shift time and to reduce torque shock to a sufficient degree.
It is to be noted that torque of an output shaft of a transmission and a running gear ratio, namely, a ratio of velocity on the input side to that on the output side of the transmission mechanism, vary during shifting as shown in FIG. 4 so that the running gear ratio should not vary for a transient period of time due to delay in a supply of oil pressure and in changes of revolutions of a turbine after a signal for instruction for shifting has been given. In this specification, this transient period of time is called "first region". After the transient period of time, namely, after the first region, the running gear ratio is caused to change and reach a predetermined gear ratio for the speed range sought to be shifted. In this specification, this period is called "second region", and a period of time after the second region, that is, after completion of the shifting, is called "third region". It is to be understood herein that, in the first region, although no running gear ratio, namely, no revolutions of the turbine is changed, torque of the output shaft is changed so that a change in the torque of the output shaft should be regulated to an appropriate extent and that, in the second region, revolutions of the turbine are changed in association with a change of the running gear ratio so that the change in the turbine revolution is required to be adjusted to an appropriate level.
Although a manner in which the torque of the output shaft and the running gear ratio vary during the course of shifting speed ranges is also changed, as have been described hereinabove, it is further understood that they vary with various other factors such as oil temperature. It is thus found difficult to adjust a variation in the torque of the output shaft and the turbine revolutions to an appropriate extent in the course of shifting merely by adjusting a time required for the shifting, as with conventional transmission disclosed in the above patent publication.